I use a rule of thumb in my book and continue to promote this. Because mini-roundabouts are so
interactive, drivers may be forgiven when occasionally
they all look at one another wondering who is going to make the first
move. This three-way stand-off is relatively common
and it has to be said that different drivers will accept
very different conditions before entering a busy
mini-roundabout.

Frank Blackmore invented a capacity rule
based on the area of the junction. He argued that the
capacity was in direct proportion to the linear size of
the junction expressed by a formula that summed the entry
widths and added a value for the square root of the
remaining area. This was based as much on his research on
the TRL test track as anything else; it suggests the
possible capacity if the area is used most efficiently.
It relates to a fundamental formula that I illustrate in
my seminars:

Q = N/t

where Q is the flow,
N is the average number of vehicles in the
system at any one moment and
t is the average time taken by any vehicle to
pass through the system.

In my capacity work I assume a reasonable
degree of one-to-one; if one driver cannot proceed then
another usually can.
This works reasonably well for small junctions and seems to relate closely to the formulas
that the Americans have used.

For a stream entering a mini-roundabout and
turning right:

QR = 1200 - QC

where QC is the circulating
traffic in vehicles per hour equivalent.

For a stream entering a mini-roundabout and
turning left:

QL = 1500 - QC

For a stream entering a mini-roundabout and
turning left and right:

QL&R = 1200 - QC

This formula results in a straight line
graph at 45°, which is close to the graphs produced by
others. There are slight end effects that we can ignore
here.
The French have identified that when trying to enter a
roundabout the traffic turning off at the first exit
(left in the UK) does have an inhibiting effect on the
next entering stream around.

This arises particularly
with a single lane approach where drivers are turning
left and right from the same lane. Entering drivers
cannot be sure exactly what the drivers from the previous
approach are doing.

In
my book are various worked examples of how I have applied my
formula.